The present invention relates in general to a method for identifying formation fractures surrounding a well casing and, more particularly, to a method for detecting the presence of hydraulically induced fractures behind such well casing.
It has long been known to acoustically log open wellbores to determine the velocities of compression ("P") waves and shear ("S") waves traveling through rock formations located in the wellbore region and tubes waves ("T") traveling along the wellbore interface. Logging devices have been used for this purpose which normally comprise a sound source (transmitter) and one or more receivers disposed at pre-selected distances from the sound source.
By timing the travel of compression waves, shear waves, and/or tube waves between the transmitter and each receiver, it is normally possible to determine the nature of surrounding rock formations including natural fracture identification. For descriptions of various logging techniques for collecting and analyzing compressional wave, shear wave, tube wave, and secondary wave data, please refer to U.S. Pat. Nos. 3,333,238 (Caldwell); 3,356,117 (Loren); 3,362,011 (Zemanek, Jr.); U.S. Pat. No. Re. 24,446 (Summers); U.S. Pat. Nos. 4,383,308 (Caldwell) and 4,715,019 (Medlin et al.); and to "The Correlation of Tube Wave Events With Open Fractures in Fluid-Filled Boreholes" by Huang and Hunter in Geological Survey of Canada, pages 336-376, 1981.
In each of the foregoing references, the acoustic waves are generated in the formation in response to an acoustic energy transmission from within an open wellbore. However, the teachings of such references are not applicable to the indentification of induced fractures in formations which are traversed by well casing, that is, well pipe cement bonded to the formation. It is therefore a specific objective of the present invention to provide for a method of logging a cased well to detect hydraulically induced fractures, in the formations surrounding the cased well.